Magnetic Resonance Imaging (MRI) systems typically include a superconducting magnet that generates a primary magnetic field within an imaging volume. To maintain superconductivity, the coils of the superconducting magnet must be kept at a very low temperature. Typically this is achieved via a coolant, such as liquid helium, with the superconducting magnet kept in a vacuum assembly to help insulate the superconducting magnet.
MRI systems also include a gradient magnet interposed between the superconducting magnet and an object to be imaged. The gradient magnet is typically pulsed so that a magnetic field varies at a relatively high frequency. To prevent the gradient magnetic field from affecting the superconducting magnet, one or more shielding layers may be interposed between the superconducting magnet and the gradient magnet. For example, a warm bore cylinder may be placed proximate an inner boundary of a housing of a superconducting magnet. In certain known systems, the warm bore cylinder is a thin, solid sleeve made of a conductive material. The conductive material allows the generally steady magnetic field of the superconducting magnet to pass thereby, but acts as a shield against the pulsing magnetic field of the gradient coil.
The pulsing magnetic field of the gradient coil results in eddy currents being created in the shielding, such as the warm bore cylinder. The formation of these eddy currents allows the warm bore cylinder to help dissipate the magnetic field of the gradient coil before the gradient coil magnetic field can affect the superconducting magnet. However, as the magnetic field increases, and the eddy currents increase, the eddy currents interact with the ambient magnetic field and cause the warm bore cylinder (or other shielding) to mechanically vibrate at an increasing magnitude. In addition to noise and potential mechanical failure, these vibrations may result in additional eddy currents and magnetic fields that propagate through the magnet's structure. Thus, the superconducting magnet becomes heated. The effects may be exacerbated by, for example, higher magnetic field, higher gradient power, and/or frequency matching between the gradient pulse and mechanical resonance modes of the conductive cylinders. This heating can affect the magnetic field produced by the superconducting magnet (and thus the quality of the image obtained), and even lead to quenching of the superconducting magnet (a damaging and potentially dangerous event including boiling of the liquid helium).